ABSTRACT This renewal application will build off of the success of the currently funded R01 by focusing on two highly prevalent, yet understudied, experiences: soldiers who have never deployed and those who are separating from the military. Substance abuse is one of the most common health problems among military personnel overall1 and tends to be even higher among reservists compared to active duty.2, 3 Research on the military has focused on the effects of deployment and combat, yet a significant proportion of reserve soldiers are never deployed.4, 5 Deployment is an important part of soldier identity; therefore, non-deployment may contribute to feelings of guilt and decreased connectedness with one's unit, which, in turn, can increase risk for negative outcomes.6 For example, in a study of 1.3 million veterans, suicide risk was higher among veterans who had never deployed.7 Separation from the military can also be stressful for service members and their spouses, and may contribute to increased substance use.8, 9 We have found cross-sectional evidence that former service members, compared to current reservists, have higher rates of substance use and dependence,10 and that both reservists who have left the military with negative health issues and their spouses are more approving of substance use than current reservists.11 Given our current project, we are uniquely positioned to examine changes in substance use and mental health among both soldiers and their spouses in our sample during the separation transition period. We propose three additional yearly assessments using an encrypted web-based survey. The scientific premise for this work is strong, building off of the on-going successful project and utilizing an adapted social ecological framework, specifically designed to address the military context.12 The use of validated and reliable measures, a proven survey methodology, and sophisticated longitudinal data analysis demonstrate a high level of rigor. We propose to accomplish the following specific aims: 1. To examine longitudinal changes in substance use (alcohol, tobacco, illicit drugs, nonmedical use of prescription drugs (NMUPD)) related to under-examined military experiences: a) never being deployed and b) the transition to civilian life following separation from the military. 2. To examine how individual-, relationship-, and community-level factors interact with these under-examined military experiences to influence changes in substance use over time. 3. To examine possible mechanisms (e.g., substance use norms and conflicts with soldier identity) linking these under-examined experiences with substance use over time. 4. To examine how changes in substance use interact with these under-examined experiences to influence intimate partner relationship functioning (verbal, physical and sexual intimate partner violence, and marital satisfaction). Given a strong foundation to build from, these new aims will, in a cost- and time-efficient manner, generate substantial new knowledge to move the field forward, thus improving outcomes for soldiers, partners, and their families.